Wubi's been dropped from 13.04. They no longer test it, and they no longer support wubi in 13.04. There was a developers mailing list post about proposing to drop Wubi, and they removed the Wubi builds for everything but 12.04.
Therefore, if I am interpreting this correctly, there are no Wubi builds for anything but 12.04. Should we consider any Wubi questions for anything that is 13.04 or newer as offtopic now, since it's no longer being supported?
For new questions that are similar to "Can I use Wubi on 13.04", Oli's answer here is most suitable.
For new questions about debugging Wubi builds that are from older beta builds that have Wubi on them for 13.04, Luis Alvarado's answer is the most suitable.
Any questions about 13.10 and Wubi, however, are offtopic and should either be closed as not supported, or Oli's solution should be utilized.